Intracellular damage in a muscular dystrophy

11 January 2018

A new publication from the laboratory of Bristol Biochemistry's Andrea Brancaccio was published in Human Mutation and chosen as the cover image.

Dystroglycan (DG) is a cell adhesion complex composed by two subunits, the highly glycosylated 𝛼-DG and the transmembrane 𝛽-DG which is particularly important for skeletal muscle stability. In skeletal muscle, DG is involved in dystroglycanopathies, a group of heterogeneous muscular dystrophies characterized by a reduced glycosylation of 𝛼-DG. The genes mutated in secondary dystroglycanopathies are involved in the synthesis of O-mannosylglycans and in the O-mannosylation pathway of 𝛼-DG while mutations in the DG gene (DAG1), causing primary dystroglycanopathies, destabilize the 𝛼-DG core protein influencing its binding to modifying enzymes.

A homozygous mutation (p.Cys699Phe) hitting the 𝛽-DG ectodomain has been identified in a patient affected by muscle-eye-brain disease with multicystic leucodystrophy, suggesting that other mechanisms than hypoglycosylation of 𝛼-DG could be implicated in dystroglycanopathies.

We have characterized the DG murine mutant counterpart (Cys697Phe) by transfection in cellular systems and high-resolution microscopy (see the cover image). We observed that the mutation alters the DG processing leading to retention of its uncleaved precursor in the endoplasmic reticulum. Accordingly, small-angle X-ray scattering data, corroborated by biochemical and biophysical experiments, revealed that the mutation provokes an alteration in the 𝛽-DG ectodomain overall folding, resulting in disulfide-associated oligomerization. Our data provide the first evidence of a novel intracellular mechanism, featuring an anomalous endoplasmic reticulum retention, underlying dystroglycanopathy.

A mutation in the adhesion molecule Dystroglycan associated to muscle-eye-brain disease with multicystic leucodystrophy results in the retention of the mutant protein within the endoplasmic reticulum (ER) of U2OS (human bone osteosarcoma) cells. The green fluorescently labelled mutant protein is imaged here by super-resolution structured illumination microscopy. The mutation results in a functionally defective and mostly internal accumulation of the protein rather than an exclusive localization at the surface of cells. The red label depicts the internal ER compartment and mitochondria of cells.